Battery with an iron electrode having a fused coating



April 21, 1970 F, ACKOWTZ ET AL 3,507,696

BATTERY WITH AN IRON ELECTRODE HAVING A FUSFZD COATING Filed Oct. :3,1968 FIG.I.

WITNESSES INVENTORS John F. Jockovi'tz and Alois Longer g g d pg gg BY47 21x4 ATTORNEY United States Patent 3,507,696 BATTERY WITH AN IRONELECTRODE HAVING A FUSED COATING John F. Jackovitz, Monroeville, andAlois Langer, Forest Hills, Wilkinsburg, Pa., assignors to WestinghouseElectric Corporation, Pittsburgh, Pa., a corporation of PennsylvaniaFiled Oct. 2, 1968, Ser. No. 764,458 Int. Cl. H01m 43/04 US. Cl. 136-25Claims ABSTRACT OF THE DISCLOSURE A battery is made containing at leastone positive and one negative electrode plate with electrolytecontacting the plates, the negative electrode plate contains an activematerial comprising iron particles having a fused coating thereon, ofelemental sulfur, selenium, or tellurium in the range of 5-20 percent ofthe weight of the iron particles.

BACKGROUND OF THE INVENTION This invention relates to a new and improvedmethod of combining sulfur, selenium, or tellurium additives with ironoxides and/or iron oxide hydrates, for use as active electrode material.-It solves problems of iron oxide-additive separation during the aqueousslurry impregnation of battery plaques.

It is generally known that an iron battery electrode plate in an alkalielectrolyte functions because of oxidation of metallic iron tohydroxides or oxides of iron or both. The exact compositions of theoxidation compound or compounds are not completely known. Although theformation of Fe(OH) Fe O and/or Fe(OH) has been postulated, Fe O seemsto be the most probable predominant compound.

The iron battery electrode plate is composed of a finely divided ironoxide powder, deposited or impregnated in a supporting plaque andcompacted to a desired density. When pure iron oxide powder is used asan electrode active material in an alkali electrolyte, a battery haslimited utilization due to the rapid formation of a passivating film onthe iron surface.

To promote the charging of the compacted powder as well as to facilitatethe discharging of the electrode plate, a reaction promoting additivefor the electrode active material is needed. Such an additive shouldhave the ability to activate the entire substance, though present inrelatively small amounts. The properties of such an additive shouldapproach those of a transfer catalyst, causing the breakdown of anyprotective film and thus keeping the iron active material surface in anactive state.

The addition of sulfur or sulfur-bearing materials, such as FeS, asadditives to iron oxide powder is known in the art. Such materials haveheretofore been incorporated throughout the iron oxide in small amounts,in an intimate mixture rather than a coating. The separate phases ofiron oxide powder and sulfur or sulfur-containing salts in intimatemixture, are subsequently converted to a slurry or paste by the additionof water. This slurry is applied to a fibrous plaque where it is pressedto the desired thickness.

Since the most convenient preparation of the iron electrode plate isimpregnation with an aqueous slurry of 3,507,696 Patented Apr. 21, 1970fine particles (about 200 mesh or 74 microns diameter), it is necessarythat the sulfur and iron oxide active material components do notseparate when water is added. With standard mixing techniques, suchseparation does occur with resulting loss in utilization of activematerial and deterioration of battery life.

It has also been found that sulfur bearing materials decompose onprolonged cycling. For example, if R28 is used as an additive, theelectrode materials will eventually fail due to slow electrolyticdecomposition of FeS, with resulting loss of sulfide ion from the ironelectrode region causing subsequent decrease in utilization of theelectrode.

SUMMARY OF THE INVENTION Accordingly, it is the object of this inventionto provide new and improved active materials for use on a battery plateby fusion coating elemental sulfur, selenium, or tellurium onto ironoxide and/or iron oxide hydrate particles.

Our invention accomplishes the foregoing object by fusing elementalsulfur, selenium, or tellurium to the iron oxide particle surface, sothat subsequent separation of the phases cannot occur during thepreparation of the electrode plate.

Comprehensive investigation indicates that no chemical change of eitherconstituent occurs during the fusion process. Plates prepared in such amanner also show longer lifetimes than plates prepared with sulfides asadditives.

BRIEF DESCRIPTION OF THE DRAWING For a better understanding of theinvention reference may be made to the drawings, in which:

FIGURE 1 shows a perspective view of a storage battery plate; and

.FIG. 2 shows a magnified view of the active material structure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In essence, a battery plate maybe generally indicated at 10 in FIG. 1 and includes a supporting metalfiber plaque 11, a body of impregnated active material 12, and a strap13 having a lead tab 14. The battery plate 10, is intended for use in asuitable electrolyte such as 25 to 40 weight percent potassiumhydroxide.

FIGURE 2 shows the active material 'of this invention, composed of ironoxide and/or iron oxide hydrate particles (the iron particle component)20, substantially fuse coated with elemental sulfur, selenium, ortellurium 21. The iron oxide particles can be spherical or of irregularshape.

The supporting plaque of FIG. 1, can be composed of a conductivematerial such as carbon or metal. It must be of a highly porousconstruction. This may be provided by a mesh or grid of fibrous strandswhich are randomly disposed either in a woven or in an unwoven texture.For example, the plaque structure 11 may be composed of a mesh of finenickel fibers which are compacted or pressed together to a density ofapproximately 10% of theoretical with about porosity. Referring again toFIG. 1, the plaque 11 is secured at its upper end to the strap 13 whichis composed of a conductive material, which, for purpose of illustrationmay be a crimped member at the upper end of the structure 11 to hold thesame in place and to provide good electrical contact. The lead tab 14-is an integral portion of the strap extending upwardly therefrom in aconventional manner. The top edge of the plaque could also be coined toa high density. The lead tab could then be welded onto the coined edge.The body of active material 12 is disposed on and within the intersticesof the supporting plaque. The active material is composed of a mixtureof particles of average size between about 37 and 74 microns, which havebeen applied on the plaque by a filtration process in the form of anaqueous slurry, which is compressed to the desired thickness andsubsequently dried.

The body of active material can be prepared by initially mixing a slurryof finely divided particles having an average particle size ranging fromabout 400 to 200 mesh (37 to 74 microns). The active particles includeat least one oxide of iron or iron oxide hydrate and the additive. Theiron particle component can include ferrous oxide (FeO), ferric oxide(Fe O ferroso-ferric oxide (FeO Fe O or Fe O hydrated ferric oxide andmixtures thereof in the form of fine precipitates.

The preferred and most practical additive is sulfur, of about 99 percentpurity, in the form of elementary flours of sulfur, colloidial sulfur,or as pure sulfur particles obtained by evaporation of a solutioncontaining molecular sulfur. The sulfur additive is dry mixed with theiron component particles followed by fusion of the components in acontainer at 100 C. to 150 C. High purity selenium and tellurium canalso be used as the additive in this invention. Selenium is available aseither pellets, stick, or shot at 99.9 percent purity. Tellurium isusually marketed with a purity of over 99.7 percent with most of theimpurity as selenium. The additive should be present in the range ofabout 5-20 percent of the weight of the iron particles.

The iron oxide compound used is commercially known as Mapico Black, asynthetic magnetite containing approximately 22% ferrous oxide (FeO) and77% ferric oxide (Fe O It is prepared by precipitating ferrous hydroxidefrom an iron salt solution and then oxidizing with air under rigidlycontrolled conditions to the desired degree of oxidation. The partiallyoxidized precipitate is dried in a nitrogen atmosphere to prohibitfurther oxidation. The compound is stable and easily handled at roomtemperature without fear of oxidation of the iron (II) sites in thelattice.

It has been our experience that surface impurities, initially present onthe iron oxide particle surface (less than 1%), do not affect thequality of the final electrode material if the fusion procedure isfollowed carefully. If the fusion vessel lid is not completely tight,allowing gas leakage, then the fusion process serves both to removeimpurities from the iron oxide particle surface and to allow sulfurcoating of the iron oxide particles.

If preheating the Mapico Black is a necessity, then care should be takento maintain the temperature below 100 C. while restricting the flow ofoxygen around the sample.

The blending process serves only to mix the two components (ironparticles and additive) uniformly. The heating temperature is the mostcritical variable in the fusion coating process and will be dependent onthe melting point of the additive.

Samples analyzed show that an average 3% reduction in iron (II) contentoccurs from initial Mapico Black to final sulfur fuse coated product.These same analyses show only trace amounts (less than .1%) of sulfideor sulfate sulfur. Thus, the fusion process of this invention does notinvolve a chemical reaction between the additive and oxide but a moresimple physical process whereby the irregular surface of the iron oxideparticles is fuse coated with the additive.

In the case of sulfur, if the temperature is kept below the meltingrange of flours of sulfur (less than about C.), then the fusion processcannot occur. Separation of the iron oxide and sulfur phases will occurif the non-fuse coated material is left standing in water. Thetemperature range for fusion of sulfur is from about 100 C. to 150 C. Atits melting range (-116 C.) and a little above, sulfur is liquid,yellow, transparent and mobile. A transparent coating is thus formed ona surface of the oxide particles which does not detach on prolongedexposure to warm water. If the temperature of fusion exceeds 150 C. theliquid sulfur turns dark brown and becomes increasingly viscous as thetemperature is further raised causing serious complications which causepoor electrochemical utilization of the active material.

The purpose of the additive is to prevent passivation of the iron duringdischarge and to create favorable conditions for an effective acceptanceof charge. A small amount of sulfur, selenium, or tellurium with FeO, FeO or Fe O and/or the iron oxide hydrates such as Fe O -H O, apparentlypromotes a greater degree of disorder in the crystal structure andthere-by enhances diffusion and electrical conductivity of the material.Such additives also influence the hydrogen overvoltage during chargingand the uptake of hydrogen by the iron. By such a combined action, theadditives influence the rate and completeness of conversion of the ironoxide particles during charging. When an electrode is prepared, it is inthe discharged condition. It must then be charged by passing a currentto convert the iron oxide and iron oxide hydrates to iron metal. Thepresence of an additive enhances the acceptance of the charge as well aspreventing the passivation of the iron upon subsequent discharge of theelectrode. The additive apparently catalyzes the reduction of the ironoxide to metallic iron by inhibiting the formation of hydrogen moleculeson the oxide surface. If an additive is not present nearly all of theelectrical energy is transformed into hydrogen molecule formation.

EXAMPLE I Sixteen pounds of 230 mesh Mapico Black (22% FeO, 77% Fe O asreceived from the Columbian Carbon Co., was blended dry with 600 gramsof flours of sulfur in a Readco AK2 electric mixer for two hours. Duringthis blending process, the temperature of the iron oxide-sulfur mixtureincreased to 40-45 C. due to heat generated by friction. While stillwarm, about four pounds of the mate rial was placed into a Teflon linedstainless steel container and closed with a Teflon lined steel lid. Thelid was held in place by bolts extending from a steel base plate underthe container and fastened by washers and nuts. These nuts on the lidwere only made finger tight so as to allow gas leakage.

To sulfur-fuse, the closed container was placed in an oven at C. for 20hours and then cooled to room temperature while still closed. The activematerial was then broken up in a grinder. This material was passedthrough a 200 mesh screen, all particles being below 74 microns in size,and packaged into polyethylene packets for storage.

The container used for the fusion process had a liquid volume of aboutone gallon. The Teflon lining protects the container walls from slowattack by sulfur or possible reaction by-products.

The small, almost indetectable, amounts of sulfide or sulfate sulfurpresent in the final material are due to reaction of sulfur with theatmosphere of the closed container rather than internalreduction-oxidation with the iron oxides. Past experiments show that, ifthe reaction is carried out in a nitrogen atmosphere, no sulfide orsulfate are formed.

Electrochemical test data was obtained for full size battery plates (55square inches). One plate was prepared using aqueous slurry impregnationof sintered nickel fiber plaques with our prepared iron-sulfur activematerial. The other plate was impregnated with charged nickel hydroxide.The results were as follows:

TABLE 1 Sample 1 2 3 (1) Analysis of iron and sulfur on the ironelectrode plate:

(A) Total sulfur (wt. percent) 9. 8 8. 6 6. 8 (B) Total iron (wt.percent) 62.0 63.0 64. (C) Sulfate (wt. percent). 0.8 0.8 0.7 (D)Sulfide (wt. percent) 02 02 02 (E) Oxygen (wt. percent) 27. 3 27. 28.4(2) Electrochemical data: 1

(A) Median output for iron plate (amp-lm/ gm. iron-sulfur activematerial). 0. 50 0. 46 0. 49 (B) Wt. iron-sulfur active material loaded(gm./55 sq. in.) 55. 5 53. 5 53. 0

Data for single nickel-iron plate system in 25 wt. percent KOHelectrolyte. Drain rate was 50 ma./cm.

Our active battery material did not separate during the aqueous slurryimpregnation of the plaques. They showed very high capacity after a fewcycles and maintained this capacity on prolonged testing. One cellsubject to such testing underwent 250 cycles with no apparent signs ofdecay. The active material of this invention can be used in negativeiron plates for batteries in numerous battery combinations as forexample one plate of a nickel-iron battery. The ratio of sulfur and theother additives to iron oxide for acceptable utilization of the ironplate is in the order of 5 to 20% of the weight of the iron oxide.

We claim as our invention:

1. A battery containing at least one positive and one negative electrodeplate, an electrolyte contacting the plates, the negative electrodeplates containing an active material comprising iron particles selectedfrom the group consisting of iron oxide, iron oxide hydrate, andmixtures thereof, the iron particles having a fused coating of additivethereon, said additive being selected from the group consisting ofelemental sulfur, selenium and tellurium, said additive being present inthe range of about 5-20 percent of the weight of said iron particles.

2. The battery of claim 1 wherein the negative electrode plate containsiron particles selected from the group consisting of FeO, FeO O Fe O FeO .H O and mixture thereof.

3. The battery of claim 1 wherein the negative electrode plate containsiron particles having an average particle size range of between about 37and 74 microns.

4. The battery of claim 1 wherein the neagtive electrode plate comprisesa supporting metal fiber plaque containing active material.

5. The battery of claim 1 wherein the negative electrode plate comprisesa supporting porous nickel fiber plaque containing active material.

References Cited UNITED STATES PATENTS 6/1930 Nordlander 106-70 1/1959Moulton et a1. 13625 UNITED STATES PATENT OFFICE CERTIFICATE OFCORRECTION Patent No. 3,507,696 Dated April 21, 1970 Inventor) John F.Jackovitz and Alois Langer- It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 3, line 26, the word "flours" should read flowers Column line 43,the word "flours" should read flowers Claim 2, column 6, line 10, theformula "FeO O should read Fe O Claim 4, column 6, line 15, the word"neagtive" should. read negative SIGNEB AND SEMED I .Attest:

Mn I. E. W, m. I Gomissionar of Patents I FORM USCOMM-DC 60376-P69 a U.SGDVEINMNT PRINTING OFFICE; IQ. D 3'J3l I I

